1. Field of the Invention
The present invention relates to material for ceramic ball that is used for manufacturing for instance a bearing and a method of manufacturing a ceramic ball.
2. Description of the Related Art
A ceramic ball is used for a bearing or the like. Being used in a rotating state, the ceramic ball is important to be even in its surface and to be close to a complete sphere in its shape.
Usually, the ceramic balls are manufactured in the following ways. Raw material powders of various kinds of ceramics such as silicon nitride and zirconium oxide are mixed and milled together with sintered additives and additives, followed by granulating in terms of a spray-dryer or the like. The granulated powder is press molded into molded bodies of spherical shape, further the molded bodies being sintered to form material for ceramic ball, as demands arise surface finishing being applied to manufacture the ceramic balls.
In the case of the existing material for ceramic ball, as shown in FIG. 3, first, powder is input into a space between an upper die 6 and a lower die 7, followed by compressing the upper and lower dies 6 and 7. Thereby, the powder is press molded into a molded body. Then, the molded body is sintered to obtain material for ceramic ball.
However, when manufacturing a bearing ball in terms of the press molding like this, in order to protect the dies, between an end portion 8 of the upper die 6 and an end portion 9 of the lower die 7 a gap must be disposed to perform press molding.
Accordingly, during press molding, the powder enters into the gap between the end portions 8 and 9 of the dies. As a result, as shown in FIG. 4, along a perimeter of the material 1 for ceramic ball, a belt-like portion 3 is formed in ring. It is usual to remove the belt-like portion 3 by polishing or mirror polishing. In order not to form the belt-like portion 3, it is considered to press the upper and lower dies to come into contact, but due to applied pressure the dies may crack or tip to result in consuming rapidly the dies.
In the existing material for ceramic ball, due to formation of the belt-like portion 3 along the perimeter of the complete sphere, it has a shape in which the belt-like portion 3 is projected from a ball. The projected portion, during polishing, comes into contact with abrasives, abrasive grain or the like to wear and damage these.
That is, as shown in FIG. 4, because a diagonal diameter R1 of the belt-like portion is usually longer than a polar diameter R2 of the spherical portion, the belt-like portion mainly comes into contact with abrasives, abrasive grain or the like to wear these.
As hardness of ceramics become higher, wear of the abrasives due to the belt-like portion on the surface of the material for ceramic ball like this becomes larger. Accordingly, members such as abrasives, abrasive grains or the like used in polishing or mirror polishing cannot be used for long.
In particular, in one of which hardness is high like silicon nitride and diameter is small, an influence of the belt-like portion on the abrasives becomes further conspicuous, resulting in even an economical problem.
Japanese Patent Laid-open Application No. SHO 63-57204 discloses a method of manufacturing a spherical body where, before sintering, chamfering is applied to a cylindrical powder molded body. In this method, in a state where the powder molded body is easy to shape, the chamfering is applied to shape into a nearly final shape. Accordingly, the processing after the sintering can be made scarce. However, the powder molded body, being very low in its hardness, tends to become damaged during the processing. In addition, whereas due to a shorter polishing time after the sintering the influence on the abrasives or the like can be lessened, a step of shaping the powder molded body is additionally required. As a result, a remarkable improvement in manufacturing efficiency cannot be expected.
Japanese Patent Laid-open Application No. HEI 2-214606 discloses that a length of a non-spherical portion sandwiched between spherical portions at both ends of a molded body is made 50 to 90% of a distance between apexes of the spherical portions, thereby a density distribution of the sphere can be made constant. However, even in the above method, the belt-like portion is not yet avoided to form, and, during polishing, the belt-like portion promotes the consumption of the abrasives.
From the above, material for ceramic ball that does not require a new step of manufacturing such as that of the processing after molding and can suppress the consumption of the abrasives in the processing after sintering is demanded.
An object of the present invention is to provide material for bearing ball. The material can be processed more easily than the existing material for ceramic ball can. The material enables to suppress wear and damage against abrasives or the like used in surface finishing the material for ceramic ball.
Another object of the present invention is to provide a method of efficiently manufacturing a ceramic ball while suppressing wear and damage of abrasives or the like.
The material for ceramic ball of the present invention, at both ends of which has spherical portions of approximate sphere and, at the center of which has a belt-like portion over an entire circumferential direction. In the material, a difference between a diagonal diameter of the belt-like portion and a polar diameter of the spherical portions is characterized to be 100 xcexcm or less.
In the present material for ceramic ball, a height of the belt-like portion from the spherical portion is preferable to be 1 mm or less.
In the present material for ceramic ball, a width of the belt-like portion is preferable to be 5 mm or less.
The material for ceramic ball of the present invention like this can be used for material for bearing ball for instance. When using in such an application, one essentially consisting of silicon nitride is preferable. Further, when using as material for bearing ball for instance, hardness of the present material for ceramic ball is preferable to be 1400 or more in terms of Vickers hardness.
The present method of manufacturing a ceramic ball comprises a step of press molding and sintering, and a step of polishing the material for ceramic ball. In the step of press molding and sintering, first the powder essentially consisting of silicon nitride is molded into a shape that has spherical portions of approximate sphere at both ends and a belt-like portion formed over an entire circumferential direction at the center portion. Thereafter, the molded powder is sintered to manufacture material for ceramic ball.
In the present material for ceramic ball, the difference between a diagonal diameter of the belt-like portion and the polar diameter of the spherical portion is set at 100 xcexcm or less. Thereby, the belt-like portion is suppressed from projecting from the spherical portion. By forming into such shape, the belt-like portion of the material for ceramic ball and the polar portions of the spherical portions come into average contact with the abrasives or the like. Accordingly, different from the case of the existing material, the wear and damage of the abrasives due to the contact with the belt-like portion can be suppressed.
Further, in the present material for ceramic ball, a height of the belt-like portion from the spherical portion is set at 1 mm or less. Thereby, during the polishing, the wear of the abrasives during the polishing can be suppressed.
Still further, in the present material for ceramic ball, a width of the belt-like portion is set at 5 mm or less. Thereby, the wear of the abrasives during the polishing can be further suppressed.
Further, the present material for ceramic ball can be used as material for bearing ball. Thereby, the bearing balls excellent in rolling life or the like can be easily manufactured.
The present material for ceramic ball can essentially consist of silicon nitride. Thereby, the bearing ball having excellent mechanical strength such as Vickers hardness of for instance 1400 or more and excellent in the rolling life or the like can be manufactured.
Still further, by use of the present method of manufacturing ceramic balls, the ceramic balls of excellent mechanical strength can be efficiently manufactured without unnecessarily damaging the abrasives or the like.